text: Most complexes containing the NO ligand can be viewed as derivatives of the nitrosyl cation, NO. The nitrosyl cation is isoelectronic with carbon monoxide, thus the bonding between a nitrosyl ligand and a metal follows the same principles as the  bonding in carbonyl complexes. The nitrosyl cation serves as a two-electron donor to the metal and accepts electrons from the metal via back-bonding. The compounds Co(NO)(CO) and Ni(CO) illustrate the analogy between NO and CO. In an electron-counting sense, two linear NO ligands are equivalent to three CO groups.  This trend is illustrated by the isoelectronic pair Fe(CO)(NO) and [Ni(CO)]. These complexes are isoelectronic and, incidentally, both obey the 18-electron rule. The formal description of nitric oxide as NO does not match certain measureable and calculated properties. In an alternative description, nitric oxide serves as a 3-electron donor, and the metal-nitrogen interaction is a triple bond.